One form of operating procedure for shaping hollow-section workpieces involves introducing the workpieces to be shaped into a pressing mold having an internal contour which corresponds to the workpiece shape to be produced. Oil or a water-oil emulsion is then pressed into the workpiece, under high pressure, by a pressure-generating system such as a pressure cylinder unit. When a sufficiently high pressure is applied within the workpiece, the desired workpiece shaping effect is then achieved by virtue of the outside surface of the workpiece being caused to bear against the internal contour of the pressing mold. However, the use of liquid or gaseous pressure agents in that operation gives rise to sealing problems because the hollow-section workpieces, for example of aluminium, suffer from tolerances so that continuous adaptation of the sealing region, in relation to the pressing mold, is required. It is scarcely a viable proposition to use flexible seals at very high pressures, particularly when more elevated temperatures are involved. There is also the consideration that the open involvement with oil also gives rise to high levels of expenditure in regard to environmental aspects and regulations relating thereto.
It is also already known from `Industries-Anzeiger, Essen` No 37, May 8th 1951, pages 393 through 396, and DD patent specification No 25 188, to use solid matter particles or elements such as sand or small balls, as a pressure medium for producing a pressure within a hollow-section workpiece to effect shaping thereof. In those procedures however the degrees of shaping deformation of the workpieces that can be achieved and the quality of shaping are generally insufficient, in particular because when the high pressures required are used the solid particles tend to jam together and then the fact that the solid particles can no longer move to a sufficient extent means that no further shaping of the workpiece can be achieved.